Electrochemiluminescence of rubrene at a zinc oxide electrode

The electrochemiluminescence (ecl of rubrene at a zinc oxide electrode has been studied with regard to the possibility of the formation of excited species via a heterogeneous electron-transfer reaction. Electrochemiluminescence has actually been observed, the spectrum of which coincides with the fluorescence spectrum of rubrene. However, the emitted light does not occur at potentials where rubrene radical cations are reduced but at potentials where radical anions are formed at the ZnO electrode. From this fact it is concluded that the emission observed originates from homogeneous electron transfer between radical cations and anions but not from formation of excited species via heterogeneous electron transfer. On the base of an energy diagram derived from the flat-band potential of ZnO in contact with a non-aqueous electrolyte and from electrochemical and photochemical data of rubrene the underlying processes are discussed qualitatively.     

Electrode behaviour of nickel in HCl-dimethylsulphoxide solutions—I. The cathodic evolution of hydrogen

The electrochemical reduction of the solvated hydrogen ion dissolved in DMSO as HCl was investigated from 25 to 45°C, at different concentrations and ionic strength. Stationary and non-stationary techniques were employed to obtained the kinetic parameters. The results were interpreted with a reaction scheme involving an initial electron transfer as rds, probably followed by an electrochemical desorption step.     

A novel donor-acceptor polymeric electrochromic material containing carbazole and 1,8-naphtalimide as subunit

We report here the synthesis of a novel polymeric electrochromic material containing carbazole (Cbz)-donor and 1,8-napthalimide-acceptor as subunit. The band gap E_g was measured using UV-vis spectroscopy and compared with that obtained by cyclic voltammetry (CV). Due to intramolecular electron transfer from Cbz-donor to 1,8-napthalimide-acceptor, the fluorescence quenching was observed. When the spectro-electrochemical and electrochromic properties of polymer film were investigated, various tones of green color were obtained on the polymeric film. In the positive regime, the polymer film obtained thereby is dark green resulting from the association of carbazolylium cation radicals at oxidized state and then it can be bleached by electrochemical reduction. Besides, in the negative regime, yellowish green color of film converted to blue attributed to reduction of the 1,8-napthalimide moiety. Finally, the polymeric electrochromic exhibits multi-electrochromic behavior, high redox stability, high coloration efficiency and reasonable response time.     

Redox properties and bond dissociations energies of phenoxyl radicals

Reduction half-wave potentials of several phenoxyl radicals have been measured by photomodulated voltammetry in acetonitrile. Two of the investigated radicals exhibit a reversible behavior, but for the others a rather quasi-reversible nature of the heterogeneous electron transfer has to be considered. The measured reduction half-wave potentials are within 70 mV of reported values for formal one-electron potentials of the phenolate-phenoxyl couples. That is why one can assume that the values of half-wave reduction potentials are close to the standard potentials of the electrochemical reactions. Differences of the bond dissociation energy (BDE)(OH), for the substituted phenols relative to the BDE(OH) of the phenol were calculated and compared with corresponding data in the gas-phase.     

Naphthoylenebenzimidazolone dyes as one-component photoinitiators

The appropriate substituents have been added to a 7H-benzimidazo[2,1-a]benz[de]isoquinolin-7-one skeleton in order to obtain a one-component initiator system. The absorption, fluorescence, and electrochemical properties of these naphthoylenebenzimidazolone dyes and their usefulness as visible light initiators of free radical polymerisation of trimethylolpropane triacrylate were examined. The initiation efficiency of these dyes was compared with that of the parent 7H-benzimidazo[2,1-a]benz[de]isoquinolin-7-one in the presence of a coinitiator. Results show that these bifunctional dyes are able to initiate polymerisation without an additional coinitiator. Photopolymerisation studies and the relationship between the rate of polymerisation and the free energy change for electron transfer revealed that, depending on the concentration, initiating radicals are generated via intra- and intermolecular electron transfer.     

Heterogeneous and homogeneous electron transfer in ambient temperature 1-alkylpyridinium chloride—aluminium chloride melts

1:2 molar ratio 1-methylpyridinium chloride—aluminium chloride and 1:2 m/o 1-butylpyridinium chloride—aluminium chloride melts offer a suitable solvent/supporting electrolyte medium for electrochemical studies of organic as well as inorganic solutes. Novel radical cation formation due to homogeneous electron transfer has also been observed for aromatic compounds having an oxidation potential more negative than + 1.5 V vs sce. Formation of the radical cations via homogeneous electron transfer has been confirmed by electrochemical, uv-vis spectroscopic, and esr techniques. The cation formation depends on the melt acidity, ie the 1-alkylpyridinium chloride—aluminium chloride ratio. These observations indicate that the 1:2 molar ratio 1-alkylpyridinium chloride—aluminium chloride melts are far more acidic than the corresponding 1:2 m/o alkali halide—aluminium chloride fused salt systems.     

The synthesis, spectroscopic and electrochemical properties, and application of new dyeing photoinitiator systems for acrylate monomers polymerization

New symmetrical bicationic polymethine dyes were synthesized and their spectroscopic and electrochemical properties were described. The bichromophoric dyes (benzothiazole, benzoxazole, indolinium derivatives) were investigated as sensitizers in the free radical photopolymerization initiated by their borate salts. The obtained kinetic results shown that bicationic polymethine dyes as the organoborate salts are much efficient photoinitiating systems of acrylate monomers polymerization than monocationic parent dyes. The rate of polymerization depends on ΔG_ET of electron transfer from borate anion to the excited singlet state of bicationic polymethine dye. The relationship between the rate of polymerization and the free energy of electron transfer process shows the dependence predicted by the classical theory of electron transfer.     

Approximation of chronopotentiometric responses by orthogonal collocation

High accuracy simulations of chronoamperometric responses are demonstrated for several electrochemical mechanisms. Derivations of the discretized equations for the simple electron transfer and ec mechanisms are given. The equations for other mechanisms are tabulated.     

A single two electron transfer or high disproportionation: Electrochemical reduction of dinitrobibenzyl

Further electrochemical investigation and analysis of electrochemical data on 2,2′ and 4,4′-dinitrobibenzyl showed that these compounds reduce in two 1e steps with similar formal potentials. The disproportionation constants for 2,2′ and 4,4′-dinitrobibenzyl anion radicals as calculated from electrochemical data are 0.02 and 0.32 respectively.     

Theoretical aspects of free radical decay in polyethylene and comparison with experimental results

Results of the study of stability of free radicals using electron spin resonance suggest a close relationship between the glass transition temperature (T_g) and free radical decay. In a detailed study of radicals in polyethylene (PE) [J. Polym. Sci. A2 6 (1968) 1435] in the sixties, it was concluded that the decay of radicals in solid polymers is connected to molecular mobility. In this report on molecular mobility using a Monte Carlo method we show with the example of PE that it is the motion of molecular structures, which, depending on temperature, lead to a transfer of radical centers and thus to the approach of radicals and their decay. Theoretical and experimental decay curves for PE are compared and based on their close correspondence it is concluded how the individual types of motions affect the stability of free radicals.     

Photomodulated voltammetry investigations on the benzyl radical

Benzyl radicals were generated photochemically directly in an electrochemical cell, using diphenylacetone as a precursor. The half-wave potentials and the plateau current of benzyl radical reduction and respectively oxidation were determined in acetonitrile, propylene carbonate, N,N-dimethylformamide and dimethylsulfoxide. While half-wave potentials and transfer coefficients were independent of chopper frequency and phase setting, the plateau current was markedly influenced by these parameters. The lifetime of the radical was obtained from the dependence of the phase shift of the current on the angular frequency. The influence of precursor concentration, light intensity and nature of the solvent on the radical lifetime was investigated.     

Conversion of alkyl radicals to allyl radicals in irradiated single crystal mats of polyethylene

The decay of alkyl radicals, the conversion of alkyl radicals to allyl radicals and the trapping of allyl radicals in irradiated single crystal mats of polyethylene have been studied by electron spin resonance (e.s.r.). It has been suggested that in the crystal core alkyl radicals react with trans-vinylene double bonds and are converted into trans-vinylene allyl radicals; at the crystal surface, alkyl radicals react with vinyl end groups and are converted into allyl radicals with vinyl end groups. The decay of radical pairs and the formation of trans-vinylene double bonds are discussed.     

Diazobenzo[a]fluorene derivatives as visible photosensitizers for free radical polymerization

Several benzophenazine dyes containing a diazobenzo[a]fluorene moiety have been synthesized and characterized by ¹H NMR spectroscopy and mass spectrometry (CI MS). The spectroscopic and electrochemical properties of these dyes were examined. These compounds were evaluated as potential light absorbing chromophores for free radical polymerization. The results are discussed on the basis of the free energy change for electron transfer from the diazobenzo[a]fluorene dyes to the electron donors/acceptors. The kinetic studies of the photoinitiated polymerization of trimethylolpropane triacrylate (TMPTA) using electron donors, such as phenylthioacetic acid, phenoxyacetic acid, N-phenylglycine and ethyl 4-N,N-dimethylaminobenzoate, and electron acceptors, such as 1-methoxy-4-phenylpyridinium tetrafluoroborate and 1-ethoxy-2-methylpyridinium hexafluorophosphate, have shown that these dyes are efficient photoinitiators for free radical polymerization in visible light. The heavy atoms present in the chemical structure may lead to excited triplet states within the dye facilitating electron transfer from these states.     

Effect of singlet oxygen sensitizers and quenchers on the photo-oxidation of some unsaturated polymers

The photo-oxidation of cis-polyisoprene was studied in the absence and in the presence of both singlet oxygen sensitizers (benzophenone, anthracene, rubrene) and quenchers (benzidine, diphenylamine, β-carotene). The amount of hydroperoxide which was formed at constant flow rate of oxygen during the u.v. irradiation of cis-polyisoprene increased by increasing the concentration of sensitizer as well as the number of quanta absorbed. The efficiency of the sensitizers used for the formation of hydroperoxide increased in the order: benzophenone < anthracene < rubrene. This effect was attributed to the fact that anthracene and rubrene can sensitize the photo-oxidation reaction by both physical and chemical generation of singlet oxygen while the generation of singlet oxygen is a purely physical process in case of benzophenone. In presence of quencher compounds β-carotene was more efficient than diphenylamine and the latter was more efficient than benzidine. The effect of a singlet oxygen oxidation on the different organic polymers was in the sequence: cis-polyisoprene > polybutadiene > styrene-polybutadiene > polychloroprene.     

Temperature dependence of the transfer coefficient in electron and atom group transfer processes

The Tafel “b-coefficient” for several different electrochemical processes appears to be almost independent of the temperature over wide ranges, indicative of a transfer coefficient which increases approximately linearly with increasing temperature. It is shown here that the transfer coefficient calculated on the basis of electron and atom group transfer theory does in fact rise with increasing temperature. If the effect is ascribed solely to vibrational frequency dispersion of the external solvent and quantum mechanical freezing of solvent modes at the lower temperatures, it is much smaller than the observed effects. Large vibrational frequency shifts in local modes, eg corresponding to adsorption of reaction intermediates, can, however, lead to effects comparable in magnitude with those observed. A transfer coefficient which increases significantly with increasing temperature can also arise from solvent dielectric spatial dispersion or local structural extension which becomes increasingly important with decreasing temperature.     

Electrochemical investigation of the influence of thin SiOx films deposited on gold on charge transfer characteristics

The paper reports on the investigation of the electrochemical behavior of a thin gold film electrode coated with silicon dioxide (SiO_x) layers of increasing thickness. Stable thin films of amorphous silica (SiO_x) were deposited on glass slides coated with a 5 nm adhesion layer of titanium and 50 nm of gold, using plasma-enhanced chemical vapor deposition (PECVD) technique. Scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS) were used to investigate the electrochemical behavior of the interfaces. In the case of SECM, the influence of the SiO_x thicknesses on the electron transfer kinetics of three redox mediators was investigated. Normalized current-distance curves (approach curves) were fitted to the theoretical model in order to find the effective heterogeneous first order rate constant (k_eff) at the sample. EIS was in addition used to confirm the diffusion barrier character of the SiO_x interlayer.     

A cyclic voltammetric and stopped-flow study of five coordinate hemins in noncoordinating solvent: evidence for electron transfer through the axial ligand

A series of 29 penta-coordinate, high spin hemin complexes were studied by cyclic voltammetry. Ten of these complexes were also studied by a stopped-flow method. These compounds differed only in the identity of the ligand occupying the fifth coordination site.E_{$\text{1}\text{2}$} for the Fe(III) → Fe(II) reduction was found to increase linearly with the pK_a of the axial ligand over the pK_a range 0.7–10 but no correlation between the heterogeneous rate constant, k⁰, and the pK_a of the axial ligand was observed. The dependence of the heterogeneous electron transfer rate constant on axial ligand structure was interpreted to indicate that electrochemical electron transfer occurs through the coordinated axial ligand. Stopped-flow measurements of the hydrazine reduction of a selection of these hemin complexes showed no correlation between the electrochemical reaction rate and the homogeneous reaction rate although the homogeneous reaction rate was dependent on the axial ligand.     

Self-protonation effects in the electrochemical reduction mechanism of p-nitrobenzoic acid

The electrochemical reduction mechanism of p-nitrobenzoic acid (NBA) in DMF has been investigated by cyclic voltammetry, polarography and controlled potential macroscale electrolysis. The stoichiometry of the reaction occurring at the first cathodic wave involves 0.8 Faraday and yields 0.2 moles of p-hydroxylaminobenzoic acid and 0.8 moles of the conjugate base of NBA, per mole of substrate consumed. This overall reaction is consistent with a mechanism involving four electron steps and four proton transfers from NBA to its basic reduction intermediates (self-protonation mechanism). Comparison with the voltammetric behaviour of the methyl ester of NBA supports the mechanism and allows to single out the species responsible for the successive reduction waves of NBA. The kinetic analysis of the self-protonation process in the conditions of cyclic voltammetry, allows to assign the rate determining steps of the overall reaction and to evaluate the rate constant of the proton transfer from NBA to the anion radical formed by reversible one electron transfer to the latter.     

Solvent effects on the rates and mechanisms of reaction of phenols with free radicals

The rates of formal abstraction of phenolic hydrogen atoms by free radicals, Y* + ArOH --> YH + ArO*, are profoundly influenced by the hydrogen-bond-accepting and anion-solvation abilities of solvents, by the electron affinities and reactivities (Y-H bond dissociation enthalpies) of radicals, and by the phenol's ring substituents. These apparently simple reactions can occur by at least three different, nonexclusive mechanisms: hydrogen atom transfer, proton-coupled electron transfer, and sequential proton-loss electron transfer. The delicate balance among these mechanisms depends on both the environment and the reactants. The main features of these mechanisms are described, together with some interesting kinetic consequences.     

Electrochemical and spectroelectrochemical characterization of the phthalocyanines with pentafluorobenzyloxy substituents

The solution redox properties and spectroelectrochemical investigation of the novel metal-free, zinc, nickel and cobalt phthalocyanines with tetra-pentafluorobenzyloxy substituents at the periphery were studied using various electrochemical and spectroelectrochemical measurements. Cyclic voltammetry and differential pulse voltammetry studies show that while Ni(II), Zn(II) and free-phthalocyanines give up to two reduction and two oxidation processes having ligand-based diffusion controlled reversible one-electron electron transfer characters, Co(II) phthalocyanine represents one ligand-based oxidation, one metal-based reduction and one ligand-based reduction processes having diffusion controlled reversible one-electron transfer characters. Assignments of the redox couples are also confirmed by spectroelectrochemical measurements. Reduction potentials of all complexes shift to positive potentials due to the electron withdrawing tetra-pentafluorobenzyloxy substituents compared with those of the phthalocyanines bearing phenoxy derivatives. A linear variation of the first reduction and oxidation potentials versus ze/r has been obtained for zinc and nickel phthalocyanines.